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Trail Mapping Many GPS receivers can automatically keep "track" of the path you travel. This can be useful if you need to backtrack, and most tracking receivers can also send the track data to a computer - either for storage, or to plot on a gpsMap. Making tracks is one of the more fun and useful aspects of GPS. You can see where you went, or where you went wrong, because your path is automatically recorded. Getting good tracks is more complicated than marking a waypoint, so we'll cover tracks in detail.

Track Settings A track is a series of locations and times. Some receivers can store over 1500 track points, but track memory will fill up quickly with the wrong settings. Before recording a three day backpacking trip, experiment with the receiver's track settings to find the right compromise between: the amount of time you'll be travelling, the resolution the track must have, and the available memory in your receiver. Different receivers need different settings, so we'll give some examples, but defer to the user manual for the mechanics. Available track logging modes may include: Automatic: Makes good use of available memory, but sometimes cuts corners or lacks resolution. Resolution: An adjustable automatic mode, making good use of available memory, with an adjustable "resolution" setting (in feet or meters), allowing you to trade off memory usage for detail. Time: Lets you set the elapsed time (in seconds) between recorded points, but keeps on recording, even if you stop. Distance: Lets you set the distance (in feet or meters) between recorded points, but can record unneeded points while travelling in a straight line. The "resolution" setting is usually best, since this allows you to pick the size of the features you're mapping. A smaller resolution (100 feet or less) produces a more detailed track, but will use up memory quicker. It's particularly disappointing to record the first part of a journey with a smaller resolution, only to miss the rest of it - or overwrite the first part - because the track memory is full. For long trips, pick a large resolution (250 feet or greater). The resulting path will be accurate, but corners and switchbacks can be simplified a bit. If a resolution setting isn't provided, choose between the others based on what you're doing and how much recording time you need.

Magnolia Bud

Track Logging To map a trail with track logging, be sure to clear out any old track data, then get a good "3D" navigation lock, with all available satellites locked and usable. Start recording just before you start travelling, and try to maintain 3D navigation mode throughout. If the receiver lapses into 2D mode, errors of 1/4 mile or more are possible. You can mark waypoints or use routes while recording a track. You can probably even turn the receiver off to conserve or exchange batteries, then continuing logging - check the manual. When you finish up, turn off track logging to conserve track memory. Some older receivers will "wrap around" and record over your track data if you leave them recording. Newer receivers let you choose between "Wrap", which overwrites old data when track memory is full, and "Fill", which just stops recording. To reduce the effect of GPS errors on trail mapping, you can travel a trail repeatedly, or record the same path in both directions. You get a more accurate trail, but the improvement depends on the number of times the trail is recorded.

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Tree Cover Tree cover is a special challenge to outdoor GPS use. Water in the trees absorbs GPS signals, making navigation difficult or impossible. A better "12-channel parallel" GPS receiver, and/or an external "amplified" antenna can vastly improve reception under tree cover, but you'll still lose coverage occasionally. As mentioned under "Acquiring Satellites", a 30-second message must be received from each GPS satellite before it is "locked" and usable for navigation. Moving through the forest interrupts these messages, so your receiver may never get to use an unlocked satellite. If you stand in the clear first, collecting as many satellites as possible, your receiver can use them all down the trail, even if some of them are briefly blocked. This is because the "heartbeat" of the GPS signal requires only one thousandth of a second to "peek" between the trees. If you lose navigation and need to re-acquire satellites, you may have better luck standing still. Use the satellite display, as described under "Satellite Coverage". For best accuracy, or for trails where GPS doesn't work, a hand-drawn map can supplement or replace GPS data. NEXT: Accuracy, Selective Availability, Corrections.

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